It’s an increasingly common sight in hospitals around the world: a nurse measures our height, weight, blood pressure, and attaches a glowing plastic clip to our finger. Suddenly, a digital screen reads out the oxygen level in our bloodstream.
How did that happen? How can a plastic clip learn something about our blood… without a blood sample? Here’s the trick: our bodies are translucent, meaning they don’t completely block and reflect light.
Rather, they allow some light to actually pass through our skin, muscles, and blood vessels. Don’t believe it? Hold a flashlight to your thumb. Light, it turns out, can help probe the insides of our bodies. Consider that medical fingerclip— it’s called a pulse oximeter.
When you inhale, your lungs transfer oxygen into hemoglobin molecules, and the pulse oximeter measures the ratio of oxygenated to oxygen-free hemoglobin. It does this by using a tiny red LED light on one side of the fingerclip, and a small light detector on the other.
When the LED shines into your finger, oxygen-free hemoglobin in your blood vessels absorbs the red light more strongly than its oxygenated counterpart. So the amount of light that makes it out the other side depends on the concentration ratio of the two types of hemoglobin. But any two patients will have different-sized blood vessels in their fingers.
For one patient, a saturation reading of ninety-five percent corresponds to a healthy oxygen level, but for another with smaller arteries, the same reading could dangerously misrepresent the actual oxygen level. This can be accounted for with a second infrared wavelength LED.
Light comes in a vast spectrum of wavelengths, and infrared light lies just beyond the visible colors. All molecules, including hemoglobin, absorb light at different efficiencies across this spectrum. So contrasting the absorbance of red to infrared light provides a chemical fingerprint to eliminate the blood vessel size effect.
Today, an emerging medical sensor industry is exploring all-new degrees of precision chemical fingerprinting, using tiny light-manipulating devices no larger than a tenth of a millimeter. This microscopic technology, called integrated photonics, is made from wires of silicon that guide light— like water in a pipe— to redirect, reshape, even temporarily trap it.
A ring resonator device, which is a circular wire of silicon, is a light trapper that enhances chemical fingerprinting. When placed close to a silicon wire, a ring siphons off and temporarily stores only certain waves of light— those whose periodic wavelength fits a whole number of times along the ring’s circumference. It’s the same effect at work when we pluck guitar strings.
Only certain vibrating patterns dominate a string of a particular length, to give a fundamental note and its overtones. Ring resonators were originally designed to efficiently route different wavelengths of light— each a channel of digital data— in fiber optics communication networks. But some day this kind of data traffic routing may be adapted for miniature chemical fingerprinting labs, on chips the size of a penny.
These future labs-on-a-chip may easily, rapidly, and non-invasively detect a host of illnesses, by analyzing human saliva or sweat in a doctor’s office or the convenience of our homes. Human saliva in particular mirrors the composition of our bodies’ proteins and hormones, and can give early-warning signals for certain cancers and infectious and autoimmune diseases.
To accurately identify an illness, labs-on-a-chip may rely on several methods, including chemical fingerprinting, to sift through the large mix of trace substances in a sample of spit. Various biomolecules in saliva absorb light at the same wavelength— but each has a distinct chemical fingerprint. In a lab-on-a-chip, after the light passes through a saliva sample, a host of fine-tuned rings may each siphon off a slightly different wavelength of light and send it to a partner light detector.
Together, this bank of detectors will resolve the cumulative chemical fingerprint of the sample. From this information, a tiny on-chip computer, containing a library of chemical fingerprints for different molecules, may figure out their relative concentrations, and help diagnose a specific illness. From globe-trotting communications to labs-on-a-chip, humankind has repurposed light to both carry and extract information. Its ability to illuminate continues to astonish us with new discoveries.
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Credit: Sajan Saini